Loudspeaking telephone system



`une 18, 1940. N. F. HOARD i LOUDSPEAKING TELEPHONE SYSTEM 3 Sheets-Sheet 1 Original Filed Feb.. 3.` 1937 i E5 En Q? June i8, i946. N, F, HQARD LOUDSPEAKING TELEPHONE. SYSTEM original Filed Feb. 3. 1937 3 Sheets-Sheet 2 June i8, N. EHQARD A2,205,142

LOUDSPEAKING TELEPHONE SYSTEM lOriginal Filed Feb.' 5. 1937 3 Sheets-.Sheet 5 Patented June 18, Y1940 UNITED STATES PATENT oFFlc-E LOUDSPEAKIYNG TELEPHONE SYSTEM Norman F.` Hoard, Milton, Mass., assignor to The Holtzer Cabot Electric Company, Boston,

Mass., a corporation of Massachusetts Application February 3, 1937, Serial No. 123,815

Renewed December 5, 1939 19 Claims.

With this object in view, the principal fea,

ture of the invention comprises a system in which the total or overall gain of both channels is nor` mally insufficient to permit approachto the singing level, together with means dependent on the input energies to both channels to increase the gain in lthe channel receiving the greater input and simultaneously to diminish the gain in the other channel. If the changes in gains of the two channels are in approximately inverse proportion, the operation is satisfactory, and the invent-ion comprehends such action, but for reasons which will hereinafter appear, the preferred system includes means whereby the gain of the channel handling the smaller input is diminished proportionately more than the gain in the other channel is increased, so that the overall gain under active conditions is somewhat less than when the system is inactive. pletely assures against any undesirable feedback and also suppresses any tendency towards echo effects. The gain control devices always stand in readiness to respond to inputs into either or both channels, and in that respectI the present invention is to be distinguished from systems in which one channel is completely 'disabled while the other is active. Normal conversation cannot be simulated if the inactive channel is completely disabled, since there isno opportunityfor one party to interrupt while the other is talking. According to the present invention, the parties are able to converse as freely as if speaking face to face.

In the accompanying drawings, Fig. l is a diagram of a system according to the present invention; Fig. 2 is an internal diagram of some of the elements of Fig. 1; Fig. 3 is a diagram explanatory of the operation of the bias control unit; and Fig. 4 is a diagram of the preferred system according to the present invention..

I General arrangement 'A system according to the present invention is shown diagrammatically in Fig. 1, being arl ranged for two-way loudspeaking conversation This arrangement com.

between two ofl'lces or other points in a building. In one oflice are a microphone M1 and a loudspeaker S1and in the other oice are a similar microphone M2 and a loudspeaker S2. The separation between each microphone vand its adjacent loudspeaker is a factor which has some influence on the operation of the system, as will hereinafter be described in detail.

The microphone M1 and the loudspeaker S11 are connected by aleft-to-right talking channel which includes a communication amplifier A1,

and the microphone M2 and the loudspeaker S1 are connected by an independent right-to-left talking channel which includes a communication amplifier A2. For the purpose of controlling the gains of the communication amplifiers, such ampliers are associatedwith control amplifiers des ignated C1 and C2 respectively. The control amplifiers are associated with a single gain control or bias control unit B which is connected to control thegains of the communication amplifiers. The arrangement is such that the control amplifiers supply the bias control unit with ampliiied voice currents traveling in either direction, and the bias control unit is connected in a man? ner to increase the gain of the active communication amplifier while diminishing the gain of the inactive communication amplifier to such an elitent that the singing or oscillating level is never approached.

The micro'phone M1 is connected with an input transformer lll, having the mid-point of its secondary grounded. The ground terminal and one end terminal of the secondary are connected to the input terminals l and 2 respectively of the .communication amplifier A1, while the ground terminal and the other end terminal of the secondary are connected to input .terminals 5 and 6 of the control amplifier C1. The output terminals 3 and 4 of the communication amplifier are connected directly yto the loudspeaker S2, while the output terminals 'l and 8 of the control amplifier are connected to one pair of input terminals lll,

' and indicated in Fig. 1 by primes. f"

Communication and control amplifiers The diagram of the communication amplifier Al and the control amplifier lC1 as well as of the bias control unit B (is givenin Fig. 2f.

The communication .amplifier comprises a three-stage amplifier unit having the tubes I6, I8 and 20. The input terminals I and 2 are connected with a grid Aresistor or potentiometer 22 having a variable tap 23 connected to the grid of the first tube I6. The plate of the tube I6 is resistance-coupled to the tube I8 in conventional manner with the exception that the grid leak 24 is not connected directly to ground, but connects through a wire 26 with the control terminal 9 of the amplifier. As will be later described in detail, the bias of the tube I 8 is capable of smooth or continuous variation through a wide range under the influence of the bias control unit whereby the gain of the communication amplifier A1 may be controlled. The cathode of the tube I8 is maintained at a positive potential with respect to ground through suitable resistances indicated generally at 28 and energized from a suitable direct current voltage source 38, which is purpose to be later described in detail. The ampliiied voice currents from the final stage are passed through an output transformer 38 to the out-put terminals 'I and 8 which are connected by leads 36 with the input terminals IIJ, II of the bias control unit B.

The control amplifier C2 which is not shown in detail in Fig. 2 is connected to the input terminals I8, II' of the bias control unit B. In other respects the connections of the amplifiers A2 and C2 are identical with those described for the amplifier A and C.

Bias control unit 'I'he bias control unit comprises devices acting upon differences in the amplified voice currents of the control amplifiers C1 and C2 to bias the commlmication amplifiers A1 and A2 in a difierential manner. transformer 38 connected to the input terminals I8 and II and a second input transformer 4I) connected with the corresponding input terminals I0', II',.said input terminals being connected, as described above, with the output terminals of the respective control amplifiers.

One end of the secondary of the transformer` 38 is connected by a Wire 42 with the plate of a ,diode rectifier 44 and a similar connection is established between one end of the transformer 40 and the plate of a second rectifier 46. Connected between the cathodes of the rectifiers are two series resistances 48 and 58 which at their junction are connected at 52 to a point of datum potential, indicated as ground. The resistances 48 and 5D are by-passed by condensers 54 and 58 respectively to smooth out fiuctuations in the currents rectified by the tubes. The lower end of the secondary of transformer 38 is connected by a lead 58 with the cathode of tube 48, the lead 58 including series resistances 59 and 68. Similarly, the lower end of the secondary of transformer 40 is connected by a lead 62 With 'I'he .unit comprises an input the cathode of tube 44, said lead including reconnects by a wire 68 with the Acontrol terminal Similarly,

9 of the communication amplifier A1. a lead 'I8 runs from the junction y of the resistances 58 and 60 to the terminal I2' which. as shown in Fig. 1, connects with the control terminal 9 of the communication amplifier A2. 'I'he potentials of the terminals I2 and I2' with respect to ground are determined by theV currents traversing the resistances 48, 84 and 50, 60 respectively, and these potentials control the gains of the communication amplifiers through the grid biases applied to the second stage amplifier tubes I8.

The operation of the bias control unit is explained in conjunction with the simplified diagram of Fig. 3 in which the elements are rearranged. For simplicity the by-pass condensers 54 and 56 are omitted: It will be seen that the unit comprises three parallel branch circuits, one comprising the tube 44, the secondary of transformer 38 and the resistances 59 and 60;

another comprising'the tube 46, the secondary of transformer 48 and the resistances 83 and 64; and the third comprising the resistances 48 and 58 in series with their junction grounded.

First, assuming no input into either microphone, no current fiowsin the bias control circuit and points :c and y arev at ground potential.

These points determine the biasing potentialsl on the grids of the tubes I8 of the communication amplifiers, but it will be noted that since the cathodes are maintained positive, the grids are normally at a negative potential with respect to their cathodes. As will be seen from the description which follows, the maximum overall gain of which the system is capable is obtained when the points .'L and y are at ground potential. By overall gain is meant the product of the gains of the individual communication amplifiers. It is essential that the gains be adjusted (by the potentiometers 22) so that, even with the maximum overall gain obtainable under inactive conditions, the system cannot approach the singing or oscillating level; that "is to say, the amplifier gains must be insufficient to overcome the losses, both electrical and acoustic, in the system as a whole.

Next, .a balanced condition may be assumed, in which equal voltages are induced in the secondaries of transformers 38 and 4D. This condition would require equalinputs to the microphones and would not frequently arise. Whenever'it d'oes occur, it will be seen that a rectified current simply circulates through both transformer secondaries, both rectifier tubes and both sets of resistances 59, 60 and 63, 64 all in series. No current flows through the resistances 48 and 50 and their ungrounded ends (designated at a and b) remain at ground potential. The bias points and y then assume negative potentials with respect to ground, the magnitude of these negative potentials being the voltage drops of the circulating current through the respective resistances 64 and 6I). The gain of each cornmunication amplifier is thus diminished, and the overall gain is therefore lower than under the inactive condition.

Finally, the important case of unbalanced voltages in the transformer secondaries is to be considered. This includes the practical condition of a person speaking into one microphone only, say M1. The control amplifier C1 delivers amplified voice currents to the input terminals l0, Il of the bias control unit,` and the secondary of the transformer 38 applies voltage in the uppermost branch of the bias control unit of Fig. 3.' There is no corresponding voltage induced in the secondary of transformer40. In addition to a current which may be 'considered to circulate in the rectifier circuits, there is now a current which iiows from a to b. through the resistances 48 and 50. The pointsa and b no longer remain at ground potential, but point a is raised to a positive potential and point bis lowered to a negative potential with respect to ground. Therefore,v the `point :I: which controis the bias`for the active amplifier A1 is made more positive, and the point y which controls the bias for the amplifier A2 is made more negative with respect to ground. Accordingly, the gain of the active amplifier is yincreased and the gain of the inactive amplifier is reduced. A similar action, butin the reverse direction, occurs if energy is delivered to the microphone M2 only.

The extent to which the amplifier gains are altered may be best understood by first considering that the resistances 60 and 64 are omitted, which is equivalent to connecting the bias leadsY to points a and b. Under the unbalanced condition in which one channel is active, the points a and b swing to equal and opposite potentials with respect to ground. Owing to the substan- Ibe altered in the same ratio but in opposite directions. As an example, if the gain of one communication amplifier were increased to twice its inactive value, the gain of the other communication amplifier would befdiminished to one-half of its inactive value. Thus the overall gain of the ysystem would theoretically be the same as for the inactive condition, and if the adjustmentsy were properly made to prevent oscillation under inactivevconditions the oscillating level would.

not be approached under any active condition.

Although the connection of the bias leads to points a and b is feasible. the preferred form of the invention employs the connection to the points :c and y atthe inside ends of the resistances 64 `and 60- respectively. i The purposeof these connections is to introduce a degenerative effect whereby the overall gain under active conditions is less 'than under inactive conditions.

This follows from the fact that when an unbalanced voltage is induced in one transformer. sec,- ondary, the current in the branch ofl the bias control circuit in which that secondary is included is equal to the sum of the currents in the other two branches. Thus, if aY voltage is induced in the secondary oftransformer 38 only,

more current ows through the resistance 60 than through the resistance 64. Although points v a and b'are at equal and opposite potentials with siderably less than one-half of normal. There is no disadvantage in depressing the gain of the inactive amplifier, and this arrangement, where- 'ances 60 and 64 in ,order to obtain the` proper bias potentials at and y, and also to limit the circulating current in the bias control circuit. As a typical example. the resistances 60 and 64 may be between 10,000 and 25,000 ohms each,

the resistances 59 and 63 approximately 250,000`

ohms each and the resistances 48 and 50 about one megohm each. 4 The smoothing condensers 54 and 56 should be of sufficient size, preferably of the order of one microfarad, to prevent the existence of any appreciable alternating potential difference between points a and b. Such an f alternating potential difference would constitute a coupling between the channels tending to cause the system Yto 4break into oscillation.

Time constantl In order to bridge over the normal pauses between words and sentences in conversation, it is desirable that the bias controls should not be immediately responsive to changes of input energy. Accordingly, a time constant circuit is provided for each communication amplifier. Eor the communication amplifier A1, this time constant circuit is shown as comprising' a resistance 12 which is included in the lead r2li between the terminal 9 and the grid leak 24, and a condenser 'I4 connected between the inner end of the resistance 12 and ground. Any sudden change in the potential of the point :c is therefore reflected in the biasing potential of the tube only after an interval determined by the time necessary for charging the condenser 14 through the resistance 12. This interval may be made as long or as short asldesired by a suitable choice of the values of resistance and capacitance.

A large time constant is desirable to afford a smooth output and to diminish the fluctuations `for a person of halting speech. On the other hand, the time constant should not be too large because it delays the shift from one talking channel to thev other. It has been found that a circuit Which provides for a delay of about one-half second'is most suitable,'this interval being adequate for taking care of normal pauses but without danger of -losing any appreciable part of the beginriirg` of a conversation.

VPotential limiting device I Int'will be understood that iny order to prevent distortion in any amplifier stage, the grid must be prevented from assuming a positive potential with respect to the cathode. With the arrangement thus far described, the grid of the amplifier stage I8 is capable of going positive with respect to its cathode whenever point a: (or y) assumes a positive potential greater than the positivepotential maintained on the cathode by the resistances 28. v

Toprevent the application A of a positive bias which would give'rise to distortion, the invention provides connections to the biasing leads 66 and 10 whereby current is by-passed to ground in the event that either point :c or y rises above a critical positive potential. The lead 66 is connected by a wire 18 with the plate of a diode rectifier 80 and the lead 10 is connected by a wire 82 with the plate of a diode rectifier 84. The cathodes of the rectifiers are biased positively by .a source of voltage 86 which may be included as part of the plate supply. The heaters of the diodes are not shown.

So long as neither point :c nor y aumes a. positive potential greater than the positive bias on the diodes, the latter are non-conductingl and the system operates in exactly the same manner as if the diodes were not present. Ii, however,

under 'the influence of an excessive energy input to either microphone, the point :z: (or 1l) reaches a positive potential greater than the diode bias potential86, the corresponding diode becomes conducting and current is by-passed to ground through the'lead I8 (or B2). The`diode thus forms a path to ground which is of low resistance as compared with the high resistance of the path 48 (or 50). This has the effect of holding the positive potential of point :c (or y) to a value lower than that which it would otherwise assume. Furthermore, owing to the curved characteristic of the diode, its plate resistance diminishes as the plate voltage is increased, and thus an increasing regulatory eiect is exerted for large positive potentials. With a proper setting of the biasing potential 88, the communication amplier can be prevented from going positive under substantially all conditions. Regardless of all `other factors, this control sets an upper limit on the gain of the active communication amplifier.

Since the diodes 80 and 8l operate to limit the swing in the positive direction only. theyhave no effect in limiting the negative bias potential applied to the inactive amplifier. 1

Gain regulation though useful in setting a limit of maximum gain,

usually,comes into action only under extreme conditions. Aside from this, however, there is an inherent regulation arising from acoustic coupling and existing under all conditions of use.

, The inherent regulation involves the acoustic coupling between the loadspeakers and their adjacent microphones. If energy is delivered into the microphone M1, the output of the loudspeaker S2v excites the adjacent microphone M2 at an energy leveldependent' on the relative positions of the loudspeaker and microphone. In the explanation of the bias control unit as given above,

amplifier of the active channel, but also, to a less degree, from the .control amplifier of the socalled inactive channel. Since the bias potentials depend.on a differential effect existing in the bias control unit, the gain of the active channel is reduced to a value less than thatfwhich it would assume if the regulation due to acoustic coupling were not present.

The principal factor in determining the inherent regulation is the relative disposition of the loudspeakers and microphones. If the loudspeakers are spaced a considerable distance from their adjacent microphones, the acoustic coupling and the inherent regulation are small, and the output volume changes greatly as the input energy is varied. On the other hand, ifV the loudspeakers and microphones are closely coupled, the inherent regulation is large and the output will change to a less degree as the input energy varies. In general, loose acoustic coupling will tend to give a jerky output, while close coupling will tend to smooth out variations in the energy level. A moderately close coupling is ordinarily desirable insofar as smoothness of reproduction is concerned. It must be understood, however, that f the overall gain must always be kept below that at which singing would occur, and the permissible gain can be made the greater, the less the acoustic coupling.

In general, it has been found that for most installations a satisfactory arrangement is obtained both from the standpoint of' gain and regulation, if the loudspeaker and microphone are about three feet apart and disposed directionally with their axes at an angle ofl about 60 degrees. Thus for normal use the loudspeaker, the microphone, and the-person speaking may be at the corners of an equilateral triangle about three feet on a side. The user is then in a position to face directly into either the microphone or the loadspeaker. Although this particular arrangement is satisfactory, it is not essential and the relative arrangements of the loudspeakers and microphones may be varied considerably both as to separation and direction, and the system may be properly adjusted to afford suiiicient amplification and volume regulation.

It will be seen that the regulatory feature depends primarily on the fact that the operation of each control amplifier depends only on its own input, while the operation of each communical tion amplifier depends on a differential effect inuenced by both inputs. In the example assumed above where the communication amplifier A1 is active, there is, owing to the acoustic coupling, a considerable input to the communication amplifier A2 andithe control amplifier C2. The control amplifier C2, having a fixed gain, is able to iniiuence the operation of the bias control unit. 'I'he communication amplifier A2, however, is biased for a low gain because of the differential effect and delivers only a small output to S1. It is important that no large output be delivered through .S1 to the party speaking at that point, because such an output would not only be a source of unpleasant side tones, but would also increase the inputA to M1 and thus tend to cancel a part of the desired regulatory efl'ect. In this connection,` the degenerative effect obtained by connecting the bias leads to points a: and y, rather than a and b, is of importance in diminishing the output of the inactive" loadspeaker,

without affecting the ability of the inactive cntrol amplifier to respond to any input delivered thereto.

ence, vand preferably, where the layout permits, in accordance vwith the degree of inherent volurne regulation desired. The communication amplifiers are pre-set for normal gain by adjusting the leads 23 of the first amplification stage. The gains are preferably made as large as possible,A short of the undesirable singing level under inactive conditions. If the gains are properly adjusted for no oscillation under inactive conditions, the oscillating state can never be reached because the system is arranged for degeneration under'all ther conditions. Preferably, the two communication amplifiers are set for equal gains. The gain of each control amplifier is also adjusted at the first stagev by the variable tap 33. The purpose of this adjustment is to permit varying degrees of stiffness in the system. 4If the control amplifiers are set for high gain, variations of linput cause correspondingly high voltages to develop in the bias control circuit, with consequent wide variations in the output. The system Amay then be said to have a high degree of stiffness which, if carried to extreme, would result in a jerky conversation. On the other hand, a soft control may be obtained by settingthe lead 33 for a low gain in the control amplier,

in which case the communication amplifiers will respond in a less degree to changes of input energy. Under soft control conditions the conversation is smoother but the gain in the active circuit is lower. The factor of electrical stiffness is similar in general effect to that of acoustic coupling, and the stiffness setting will usually depend on-the arrangement of the loudspeakers and microphones. The most satisfactory setting is obtained by experiment for each particular installation. As in the case of the `communication amplifiers the two control amplifiers are set for equal gains.

The foregoing equipment adjustments are made at the time of installation and thereafter no further adjustments are necessary except where changes in the operation are desired.

Modified system For practical installation the system shown in Fig. 4 is preferable to that shown in Fig. 2, although the principles upon which it operates are identical. The system of Fig. 4 will be described only as to those details in which it differs from that of Fig. 2.

The system preferably employs metal tubes, the case's of which are grounded. The system is constructed to operate from either alternating or direct current through a power supply indicated generally at 90. Aside from the ground on the tube cases, the system is ungrounded and it may, therefore, be connected to alternating or'direct current lines of suitable voltage without regard for grounding of the line conductors. The power supply 90 is connected with a resistor 92 which is tapped at various points to provide the necessary plate, grid and cathode potentials for the tubes in a manner which requires-no detailed description. The heaters of the tubes maybe connected for series operation in a well-known manner.

Aside from .the power supply, Fig. 4 illustrates only one-half the system, namely the communication ampliiier for the left-to-right channel, the corresponding control .amplifier and elements making upone-half of thebias control unit. For the` other channel the connections are identical, the supply resistor 92 being connected in an obvious manner therewith.

The first tube ||6 which is energized from the microphone M1 serves as a common first stage for both the communicationand control amplifiers. The output of the tube ||6 is connected with two potentiometers designated atl |22 and |33 respectively, the former corresponding to the communicationl amplifier gain control22 of Fig. 2, and the latter corresponding to the stiffness control 33 of the control amplifier.

The second stage of the communication amplifier designated I |8 is a multiple grid tube having a sharp cut-off grid to which the signal voltagefrom |22 is .applied and a remote cut-oit grid .to which the control voltage is applied. The control voltage is applied by a lead |26 suitably connected with the bias control unit in the same manner as the lead 26 of Fig. 2. A time constant circuit represented by the resistance |12 andthe condenser |14 is included in the bias control lead. The third stage |20 of the communication amplifier is a conventional power tube which is connected to Afeed the loudspeaker S2 throughv a. suitable transformer.

In the control ampliiier, the second stage 94, to which voltage is applied from the potentiometer |33, takes the place of the two final stages of the control amplifier of Fig. 2. The output is deliveredto a resistor |38 which takes the place of the transformer secondary 38. The resistor is connected inthe bias control unit, which includes the diode elements |44, the resistances |64, |63, and |48, and the by-pass condenser |54, all of which are identical with and arranged similarly to the corresponding parts 44, 64, 63, 48 and54 of Fig. 2. It will be understood that itis only necessary for the bias controlv unit to be energized from a point of` fixed gain, andit might be connected across the potentiometer |33 if sufficient gain were afforded by the tube ||6 alone. The bias control lead |26 is connected at the junction :c of resistances |63 and |64. A lead |52 corresponding to the ground lead 52 is connected to a suitable point of the resistor 92, thus establishing a datum potential determinative of the amplifier gains under inactive conditions.

Another pair of diode elements |84 serve the purpose of the positive potential limiting device 84 of Fig. 2, the plate being connected with the controllead |26 of the amplifier stage ||8 and the cathode being biased at a predetermined positive potential by a suitable connection 96 to the resistor 92. For convenience, the diodes |44 and |84 are enclosed ln a single case.

The other half of the system which is not shown in Fig. 4 is substantially identical to that described above. .The two halves of the bias control unit are connected together by suitable leadsy so that a circuit identical with that shown in Fig. 3 is obtained.Y Y

The system of Fig. 4 offers the advantage over that of Figs. 1,12 and 3-in reducing the number of tubes required, and also in diminishing the` v correspondingly simplied.

The operation ofthe preferred system is identical with that of the system rst described.

Conclusion interrupt while the otheriparty is talking. Con-` versation cannot be said even to approach normal if capacity for interruption is not afforded.

'Ihe ability to permit interruptions follows primarily from the independence of the communication and control amplifiers of each channel whereby each xed-gain control amplifier is responsive. only to its own input energy, while each communication amplifier operates not only in accordance with its own input, but by a differential.effect arising from. the excitation of the bias control unit from the two control amplifiers. The gain of the inactive communication amplifier may be reduced to a very small value, yet the socalled inactive control amplifier always stands in readiness to influence the bias control unit in accordance with any input Venergy delivered thereto. The silent party is not dominated by the other, but may break in at any time. If both parties continue to talk together, the gains of both amplifiers then adjust themselves to values determined by the differential effect inthe bias control unit.

It is also desirable that the' interrupting party should break in smoothly. For this purpose the control afforded by the remote cut-off grid of the tube H8 of Fig. 4 is useful. Even for large negative biasing potentials of the bias lead |26,

lthe communication amplifier is not completely cut off although its gain'may be so small that no side tones are delivered to the loudspeaker. If the party who has been silent now cuts in, the energy will rise smoothly at a rate determined by the time constant circuit. It is entirely feasible to have the tube lll cut off completely at some critical negative potential, but in such a case interruptlon by the formerly silent party will lnot result in'any output at the other end until the bias lpotential exceedsthe critical cut-off potential.

' Thus the first word or syllable may be entirely lost and the rest of the conversation will start suddenly after the critical potential has been passed. With the remote cut-off control, the volume rises smoothly and although the first word or syllable maybe of low volume,- there is no unpleasant Jerky effect.

Although the preferred embodiments of the invention have been shown and described, it will be understood thatY the invention is vnot limited to such preferred embodiments but'may be varied therefrom within the scope of the appended claims. l

Having thus described thenvention, liclaim: i

1. A two-way loudspeaking-telephone system comprising separate communication amplifying' `1 channels Vhaving gains insufficient to permit oscillation due to acoustic feed-back when the system is inactive, and means operated by differential-4 inputs tol said channels to increase the gain of the channel receiving the greater input and to reduce the gainl of the other channel, said changes of gains being in approximately inverse ratio.

2. A two-way loudspeaking telephonesystem comprising separate communication amplifying channels having gains insufficient to permit oscillation due to acoustic feed-back when the system is inactive, and means'operated. byx differential inputs to said channels to increase'the gain of the channel. receiving the greater input and to reduce the gain of the other channel, said means operating to maintain the product of the ampli- .fier gains less than for the inactive condition.

-comprising two channels, each including a communication amplifier, a gain control circuit having means responsive to the difference of the input energies of the communication amplifiers, and connections between the gain control circuit a'nd the communication amplifiers to increase the gain of-the communication amplifier receiving the greater amount of input energy and to reduce the gain of the other communication amplifier.

5. A two-way loudspeaking telephone system comprising two channels, each including a communication amplifier, a gain control circuit, means for applying to the gain control circuit voltages dependent on the input energies of the amplifiers, the gain control circuit having a portion carrying a current dependent on the difference between said voltages, and connections between the gain control circuit and the amplifiers to increase in accordance with said current the gain of the amplifier receiving the greater amount -of input energy and to reduce in at least as great a proportion the ,gain of the other amplifier.

6. A two-way loudspeaking telephone system comprising two amplifying channels, a bias control circuit having three parallel branches, means for impressing in two of'said branches voltages dependent on the energies transmitted through the respective channels, the third branch carrying a current dependent on the difference of said voltages, means for establishing a datum potential at a point of thethird branch, and connections to impress on the amplifyingchannels bias potentials determined by the voltage drops on opposite sides of the point of datum potential in the third branch.

7. A two-Way loudspeakingtelephone system comprising two amplifying channels, a biascontrol circuit having three parallel branches, means `for impressing in two ofl sald branches voltages ing a current dependent on the difference of said voltages, means for establishing a datum potential at the mid-'p'oint of the thirdL branch, whereby on passage of current through said third branch the ends thereof assume equal and opposite potentials with respect to said datum potential, and

Ameans for impressing on the amplifying channels two voltages dependent on the energies received by the respective channels, a branch adapted to carry current only when the impressed voltages are unequal, means for establishing a datum potential at a point of the branch, and connections between the bias control circuit and the channels to impress on the' latter bias potentials dependent on the voltage drops through said branch on oppo'site sides of the point of datum potential.

` 9. A two-way loudspeaking telephone system comprising a first amplifying channel and a second amplifying channel, a bias control unit having three branches, means for impressing in one branch a voltage dependent on the input energy to the first channel, means for impressing in the second branch a voltage dependent on the input energy to the second channel, means for establishing a datum potential at a point of the third branch, a bias lead for the first channel connected to a point of the second branch, and a bias lead for the second channel connected to a point of the rst branch.

10.v A two-way loudspeaking telephone system comprising a f'lrst amplifying 4channel and a second amplifying channel, a bias control unit having three branches, meansfor impressing in one branch a voltage dependent on the input to vthe first channel, means for impressing in the second branch a voltage dependent on the input to the second channel, each of said, two branches including a rectifier and a resistor in series, Q

the third branch being adapted to carry a current dependent on ,the differences of the voltages impressed in the first two branches, means for establishing a datum potential at the mid-point of the third branch, and bias connections leading from -points of the resistors/in the first two branches to the amplifying channels.

`11. A two-way loudspeaking telephone system comprising two communication amplifiers, a control amplifier associated with each communication amplifier, an input circuitv common to one communication amplifier and its associated control amplifier, a second input circuit common to the other communication amplifier and its associated control amplifier, a bias control circuit, means for connecting both control amplifiers with the bias control circuit, bias leads connecting the bias control circuit with the communication amplifiers and normally determining amplifier gains insufficient to permit oscillation, the bias control circuit having means operated in accordance with the difference of the outputs of the control amplifiers for increasing the gain of the communication amplifier receiving the greater amount of input energy and reducing in- -at least-as great a proportion the gain of the other communication amplifier.

12. A two-way loudspeaking telephone system comprising a communication amplifier, bias control means for applying to the amplifier a variable bias potential with respect to an established datum potential, and limiting means to prevent application of a bias greater than a predetermined value -to the amplifier.

13. A two-Way loudspeaking telephone system comprising a communication amplifier, bias control means for applying to the amplifier a variable positive or negative bias potential with respect to an established datum potential, and means acting upon a positive potential greater than a predetermined value to by-pass current and thereby to limit the bias applied to the amplifier.

14. A two-way loudspeaking telephone system comprising a communication amplifier, control means including a bias' lead for applying to the communication amplifier a variable bias with respect to an established datum potential, and

bias limiting means including a biased diode connected between the bias lead and the point of datum potential to by-pass current upon the occurrence of a positive bias greater than a predetermined value, thereby to limit the bias applied to the amplifier.

15. A two-way loudspeaking telephone system comprising. two amplifying channels, a bias control circuit having means for impressing therein twovoltages dependent on the energies transmitted through the respective channels, a branch adapted to carry current only when the impressed voltages are unequal, means for establishing a datum potential et a point of the branch; connections between the bias control circuit and the channels to impress on the latter bias potentials dependent on the voltage drops through said branch on opposite sides of the point of datum potential, and means connected between said connections and the point of datum potential to limit the bias potential applied to either ampli-' fier.

16. A two-way loudspeaking telephone system comprising two amplifying channels, a bias control circuit having means for impressing therein two voltages dependent on the energies transmitted through the respective channels, a branch adapted to carry current only when the impressed voltages are unequal, means for establishing a datum potential at a point of the branch,. connections between the bias control circuit and the channels` to impress on the latter bias potentials dependent on` the voltage drops through said branch on opposite `sides of the point of datum potential, and biased diode meansA between said connections and the point ofdatum potential to limit the bias potential applied to either yamplifier.

1'7. A two-way loudspeaking telephone system comprising two variable-gain communication amplifiers, a fixed-gain control amplifier associated with each communication amplifier, and

-sponslve to the difference of the input energies to two two channels, to cause variations of gain in the two channels, and connections between the channels and the gain control means to operate the latter under active conditions to increase the gain of the channel receiving the greater amount of energy and to reduce lthe gain of the other channel to an extent such that the overall gain is no greater than under inactive conditions.

19. A loudspeaking telephone system comprising two amplifying channels, both responsive at all times to` input energies delivered thereto, variable-gain means for the amplifying channels, and means operated in accordance with the difference in input energies to the channels to increase the gain of the channel receiving the greater input and to reduce the gain of the other channel.

- NORMAN F. HOARD. 

